The invention relates to reactive mufflers for internal combustion engines, and more particularly to an improved, compact resonator type muffler which creates relatively little back pressure with a high degree of noise attenuation.
Resonator type mufflers are well known. Typically tubes, called inductor tubes, have extended within a housing between two resonating chambers, with some reflection and reversal of the direction of travel as in a baffle. In the process, noise is attenuated. However, the actual physical mechanism of noise attenuation in these resonator type mufflers has not been completely understood. Precise analysis of noise attenuation of the various frequency ranges in a resonator muffler is almost impossible because the superimposed air stream has an enormous and complex effect on the results.
The diameter and length of the inductor tube together with the dimensions of the chambers determine to a large extent the frequencies that will be attenuated, i.e., small chambers and short tubes are more likely to attenuate the higher frequencies. The degree of noise attenuation (reduction in dB(A) level) at a particular frequency is influenced by the acoustical characteristics of the inductor tube. High acoustical inductance, using small tubes, has meant a high gas velocity leading to a large pressure loss, since the kinetic energy associated with this gas velocity is lost as the gas exits the tubes.
In a typical motorcycle and in some automotive designs it has been desirable for the required internal volumes to be generated by length rather than diameter. In particular on most motorcycles the chambers have been very narrow and megaphone shaped for esthetic reasons. In the past these attractive megaphone shaped mufflers have used a "glass pack" design (using fiberglass for acoustical dissipation). Since this amounted to wrapping fiberglass around a perforated straight through core, the narrow design was easy to work with. Previous resonator designs for motorcycles, on the other hand, were severely restrictive to the flow. Historically, therefore the "glass packs" have enjoyed a performance reputation while the more noise effective resonator types have been passed by.
There has always been a need for mufflers of compact design and maximum noise attenuation with a minimum of back pressure imposed on the engine. These needs have been particularly acute in the field of motorcycle noise reduction and also industrial engines. The space criteria for industrial engines are generally somewhat different from those for motorcycles, usually requiring shortness of length to avoid too much protrusion from the engine.
It is among the objects of the present invention to improve noise attenuation capability in a resonator type muffler while keeping back pressure unusually low and space requirements to a minimum.